Cellular system operators are faced with a number of problems due to the presently inefficient power amplifier (PA) implementations. Cooling is the chief concern. The inefficient PA arrangements generate a tremendous amount of heat which must be dissipated to prevent damage to the PA and to associated circuitry. Because of the large amount of heat generated, the cellular base station has significant energy demands for operation of cooling equipment. Also, the inefficiencies of the devices themselves require that they too consume excess electrical energy. In some large base station implementations, the base station may actually present electrical requirements exceeding the local electrical supply.
The problem presents itself primarily as a result of the waveform being amplified. In an IS-95 based code division multiple access (CDMA) system, for example, the amplified waveform has a significant peak to average output deviation. A typical deviation is as much as 8 decibels (dB). To ensure that the signal is amplified without clipping or otherwise distorting the waveform, the amplifier is sized to accommodate the peak output requirements resulting in loss of efficiency.
One measure of the efficiency of the amplifier is the ratio of the average amplifier output power to the average direct current (DC) input power. In a typical CDMA arrangement, or similarly in a multi-tone arrangement, the amplifier is sized approximately 7-12 dB above the average output power to prevent clipping or other distortion of the amplified waveform. In addition, the amplifier will typically operate below the average output power level about 30% of the time, and will operate at peak output only a very small percentage, less than 5%, of the time. Thus, the efficiency of the power amplifier in a CDMA arrangement is very poor and typically only about 20%. In comparison, for an advanced mobile phone system (AMPS) base station with single carrier power amplifiers, a conventional class AB amplifier may be operated at a saturated efficiency as high as 50-60%.
To improve efficiency, one would ideally like to modulate the DC input voltage to follow the required amplifier output. However, providing a voltage modulator capable of operation at the bandwidth required for IS-95, 1.25 megaHertz (MHZ), and at even larger bandwidth for proposed next generation systems, presents significant difficulties. In addition, there is excessive delay associated with detecting the output RF waveform and using this information to form a voltage modulator input signal. Thus, even if a voltage modulator capable of operating at the required bandwidth was available, it has not been possible to effectively utilize the amplifier output to provide a feedback signal to the voltage modulator.
In commonly-assigned U.S. Pat. No. 5,239,275, Leitch et al. recognized that providing a step-wise varying DC input voltage can significantly improve amplifier efficiency. As taught by Leitch et al., a baseband, input signal is observed and utilized to form an input signal to a voltage modulator, actually a step-wise voltage generator which selects one of a number of DC voltage values. This overcomes the delay problems associated with detecting the RF waveform for generating a modulation signal. Unfortunately, a suitable baseband signal is not available in many system types. Similarly, in a multi-tone arrangement where a number of signals are combined at RF prior to amplification, a baseband representation of the signal to be amplified is likewise unavailable. And, in Fourier transform matrix systems, where a combined RF signal is split, amplified through a network of PAs and then recombined to produce an output signal, a suitable signal for use in performing voltage modulation is not available.
Thus, the problem of improving the efficiency of the RF power amplifier in cellular base stations, and particularly CDMA and other multi-tone systems, remains. Therefore a need exists for a power amplifier system with improved efficiency.